1. Field of Invention
This invention relates generally to an expansible anchor and expander nail assembly insertable into a hole drilled in masonry to fasten an object thereto, and more particularly to an all-metal assembly of this type in which the expander nail is hammered into a hollow metal anchor to expand it within the drilled hole to lock the anchor therein and in which excessive hammering is not damaging to the anchor.
2. Status of Prior Art
It has long been known to secure an object to brick or other form of masonry by means of an expansible hollow anchor having a head and a divided or split body extending from the head, which body is expanded when a pin or nail is driven therein. Thus the Hottenroth U.S. Pat. No. 1,138,219 discloses an expansible hollow anchor which is inserted through a mounting hole in the object to be fastened into a hole drilled in brick or tile. When a pin is then hammered into the hollow of the anchor, it acts to spread the divided body against the wall of the drilled hole to lock the anchor therein and thereby fasten the object to the brick or tile.
The concern of the present invention is with an expansible metal anchor and nail assembly in which the anchor is formed of cast zinc or of a metal having similar characteristics. Of greatest prior art interest therefore is the commercially available anchor and nail assembly being marketed by The Rawlplug Company, Inc. of New Rochelle, N.Y. under the trademark ZAMAC NAILIN.
A NAILIN assembly is constituted by a rivet-shaped hollow anchor having a head and a split cylindrical body extending therefrom, the anchor being fabricated of a zinc alloy resistant to corrosion. The expansion device for this anchor is a nail of plated steel or stainless steel, depending on the degree of corrosion resistance required.
The NAILIN assembly is useable in concrete, block, brick, stone or any other form of masonry to fasten a fixture or other object thereto having a mounting hole. A hole is drilled in the masonry having the same nominal diameter as the cylindrical body of the anchor. The anchor is then inserted through the mounting hole of the fixture into the drilled hole and gently tapped until the head of the anchor is set tightly against the fixture. Then the expander nail is hammered into the hollow zinc anchor to expand the body of the anchor. When fully hammered in, the head of the nail which goes into a countersunk well in the mushroom-shaped head of the anchor is then flush with the anchor head.
A NAILIN anchor and nail assembly, as well as those having a similar structure, is an effective fastening device, but only when carefully installed. But when improperly installed as is often the case, this anchor assembly then has serious shortcomings. Since the expander nail is easily driven forward by a hammer, it is common practice for an installer to overdrive the head of the nail into the head of the anchor. When therefore the head of the nail is excessively struck by the hammer, the nail head advances to force zinc in the head of the anchor to extrude into the mounting hole of the fixture surrounding the junction of the anchor head and the body of the anchor. This forced extrusion develops shear stresses in the metal of the anchor within its head. As a consequence of these shear stresses, the head of the anchor will in a relatively short time period separate from the anchor body and the fixture will then become unfastened from the masonry.
Another drawback of a conventional expansible metal anchor and nail assembly is that its holding power is mainly determined by the tensile strength of the cross-sectional area of the metal body of the anchor adjacent its head. The exterior surface of the nail shank is smooth, and while this shank functions to expand the anchor body, it does not act to enhance its tensile strength. Hence the holding power of a conventional anchor is limited.
A third disadvantage of a conventional assembly is that the expander nail, when driven into the anchor, then has its head submerged in the countersunk head of the anchor; hence it cannot be withdrawn therefrom. Should it later become necessary to remove the fastened fixture from the masonry, it is not possible to do so except by drilling out the nail.
Also commercially available is a NYLON NAILIN assembly whose anchor is molded of nylon and whose nail is formed of steel or stainless steel. The drawbacks of a NYLON NAILIN assembly are similar to those encountered with an all metal ZAMAC NAILIN assembly.